In standard methods of obtaining metallic lead from concentrates, the standard procedure has been to treat the lead sulfide concentrates in a blast furnace. However, the pyrometallurgical procedure possesses many disadvantages and drawbacks. Primary among these disadvantages is that the process will result in some major pollution problems such as the generation of sulfur oxide gas along with substantial fuming. The fuming carries with it possible carcinogenic compounds which will contain lead, cadmium, etc. Therefore, it is necessary to provide improved and safer methods for obtaining metals such as lead in metallic or elemental form by methods which will not contribute to pollution of the air or will be safer to operate. The aforementioned lead smelting techniques will consist of roast sintering the lead sulfide concentrate whereby a major portion of the sulfur will be removed followed by melting in a blast furnace to obtain the metallic lead.
In an effort to alleviate the pollution problem, it is necessary to develop new processes for obtaining lead which will be competitive as an alternative to the conventional smelting practices. Prior work in the hydrometallurgical field resulted in developing a non-aqueous processing route whereby lead sulfide concentrates are chlorinated at temperatures above 300.degree. C. to produce lead chloride and volatilized sulfur. However, chlorination at these elevated temperatures will promote the formation of volatile chlorides of contaminating elements such as iron, magnesium, aluminum, silicon, zinc, as well as elemental sulfur which may be present in the lead sulfide concentrate. Other hydrometallurgical processes which have been developed include the use of ferric sulfate as a leach agent. In this method, the lead sulfide is sulfated to form lead sulfate. This step is then followed by carbonation of the lead sulfate to form lead carbonate and thereafter the lead carbonate is subjected to dissolution in hydrofluorsilicic acid for electrolysis to metallic lead. Yet another hydrometallurgical method which is developed for the recovery of lead is based on the use of a ferric chloride medium. This method involves a leaching step whereby the lead sulfide is converted to lead chloride and thereafter subjected to steps of solubilizing, crystallization and electrolysis.
The prior art which exemplifies some methods of recovering lead or other metals is exemplified by United States Ser. No. 607,287 now U.S. Pat. No. 4,024,643 in which sulfide ores are chlorinated at a temperature which is high enough to cause volatilization of some metal chlorides as well as forming gaseous sulfur chlorides. However, in such a process heat does not have to be added due to the exothermic nature of the reaction, but in order to produce elemental sulfur and to minimize the conversion of impure metals, the temperature must be controlled. The chlorinated product is then leached with water which would quickly turn into a chloride leach system containing all soluble metal chlorides to a high degree. The water in this case is a scrub solution to remove the volatile chlorides and sulfur chlorides from the off-gas stream. This, therefore, is not a wash process but in contradistinction is a leach process in which all metal chlorides which had been substantially converted are leached, leaving behind a residuum of sulfur and only minor amounts of unreacted sulfides. This leach or wash solution acts as the prime metal recovery step whereas, as will hereinafter be shown in greater detail, the wash step of the present invention has, for its primary aim, the removal of a small amount of unwanted metals that had been only slightly converted. Furthermore, due to the high temperature chlorination of the process of the patent it is, of necessity, a non-selective chlorination.
U.S. Pat. No. 1,346,642 describes a separation process for lead and zinc using the differential action of strong hydrochloric acid in an aqueous leach system. In this process, which is a wet chlorination process, the action is effected at a temperature of about 100.degree. C., the hydrochloric acid converting the sulfides, except zinc, to chlorides and the sulfur to gaseous hydrogen sulfide. This treatment is not nearly as selective as the dry halogenation step of the present process, the wet acid treatment producing a greater amount of soluble zinc chloride. Furthermore, the sulfur does not remain as an innocuous solid in this process. The lead chloride and unreacted zinc sulfide are then separated from the solution containing the soluble metal chloride impurities such as iron and copper. A water wash is used to remove the entrained hydrochloric acid leach liquor in the initial leach solids and is combined with the acid leach solution forming the next series of wash. Therefore, this cold water wash actually becomes the leach solution on recycle and, as hereinbefore pointed out, will contain a relatively large amount of impurities. Inasmuch as this wash step is a displacement step to remove all of the free hydrochloric acid, it is not intended to remove the metal impurities per se, but rather to remove the solution which would continue to leach the impurities.
Another patent which is drawn to a method for recovering values from a sulfide ore is U.S. Pat. No. 1,943,340. This patent concerns a two-stage roast of zinc concentrates wherein the first step relates to the sulfation of the concentrate and the second stage relates to a chlorination. Zinc is removed after each roasting stage or step, however, there is no wash step described in this patent. Likewise, U.S. Pat. No. 3,961,941 is drawn to a method of producing metallic lead and silver from the corresponding sulfides. The patent teaches a two-stage ferric leach-brine leach process for lead concentrates and is a wet ferric chloride conversion of lead sulfide to lead chloride. The only wash step which is described in this patent involves the use of said step to wash chloride out of the process tailings before said tailings go to disposal. In contradistinction to the prior art processes, it will be shown in the following specification that metallic lead in a purer sheet form may be produced in a simple and economical manner by a series of steps hereinafter set forth in greater detail.
This invention relates to a hydrometallurgical process for the recovery of lead. More specifically, the invention is concerned with an improved process for obtaining lead from lead sulfide sources or concentrates whereby unwanted side reactions are minimized, thereby insuring that the metallic lead which is recovered in the last stage of the process is in a relatively purer state than has heretofore been possible. As was previously discussed, lead may be recovered in an electrolysis process in which the electrolysis is effected in a molten salt stage. However, the lead which is recovered is in molten form and must be subjected to further treatment such as casting in order to recover the lead in a usable form. Likewise, another disadvantage which is inherent when utilizing an electrowinning process in an aqueous brine medium as set forth in the prior art is that it is necessary to utilize diaphragm cells and the lead which is recovered is in the form of a powder which must thereafter be melted down to ingot form in order to be more easily handled. In contradistinction to this the advantages which are present when utilizing the process of the present invention is that the lead which is obtained is in the form of a sheet on the cathode and is a dense coherent form. In addition another advantage which is found in electrowinning the lead from a hydrofluosilicic acid is that any bismuth which may still be present as an impurity is rejected, thereby permitting the lead to be recovered in a purer state.
It is therefore an object of this invention to provide an improved process for the production of metallic lead.
A further object of this invention is to provide a hydrometallurgical process for the production of relatively pure metallic lead in sheet form using lead sulfide concentrates as the lead bearing source.
In one aspect an embodiment of this invention is found in a process for the production of lead from a lead bearing source which comprises the steps of subjecting said lead bearing source to an activation or drying treatment, halogenating the treated lead bearing source, leaching the halogenated lead bearing source with brine, filtering the leached solution to separate gangue from soluble lead halide, crystallizing said lead halide and recovering metallic lead, the improvement which comprises converting said crystallized lead halide to lead carbonate by treatment with sodium bicarbonate, solubilizing said lead carbonate in hydrofluosilicic acid and subjecting the resulting lead fluosilicate to electrowinning to form metallic lead.
A specific embodiment of this invention is found in a process for the production of lead from a lead bearing source which comprises subjecting said lead bearing source to an activation treatment at a temperature in the range of from about 300.degree. to about 600.degree. C., chlorinating the activated lead bearing source with chlorine gas at a temperature in the range of from about 80.degree. to about 120.degree. C., leaching the chlorinated source at a temperature in the range of from about 80.degree. to about 120.degree. C. with a sodium chloride solution, filtering the leaching solution to separate gaugue and sulfur from soluble lead chloride, crystallizing said lead chloride at a reduced temperature, converting the lead chloride to lead carbonate by treatment with sodium bicarbonate, solubilizing the lead products in hydrofluosilicic acid to form lead fluosilicate and carbon dioxide, subjecting the lead fluosilicate under electrolysis conditions which include a voltage in the range of from about 2.5 to about 4.0 volts and current density in the range of from about 100 to about 400 amps/m.sup.2, and recovering the resultant metallic lead.
Other objects and embodiments will be found in the following detailed description of the present invention.
As hereinbefore set forth the present invention is concerned with an improvement in a hydrometallurgical process for the production of metallic lead, whereby said lead is recovered in sheet form. The feedstock which may be utilized in the present process will comprise a lead sulfide source either in the form of flotation concentrates or raw feed ores which are materially rich in lead sulfide, although it is contemplated that some of the lead may be present in the form of lead carbonate or lead oxide. In one embodiment of the invention the lead source may be activated prior to a halogenation step, said activation being accomplished by heating the lead source to a temperature in the range of from about 300.degree. up to about 600.degree. C. or more in an appropriate apparatus such as an oven. The aforesaid roast is preferably effected in an inert atmosphere or a reducing atmosphere; examples of inert atmospheres which may be employed including nitrogen, helium, argon, etc., while the reducing atmosphere may be provided for by the introduction of hydrogen or a mixture of hydrogen and carbon monoxide into the roasting zone. Generally speaking, the lead source is activated by the heat treatment at the aforesaid temperatures for a period of time which may range from about 5 minutes up to about 60 minutes or more. The velocity of the gas which provides the inert or reducing atmosphere may be relatively low in nature and need only be fast enough to purge the volatile materials from the ores as they are formed and sufficient enough to maintain the desired inert or reducing atmosphere, thus preventing any oxidative conditions which may form and which will adversely affect the subsequent halogenation of the lead source.
As an alternative step in the process of this invention, the feedstock may only be subjected to a drying operation in order to remove any water which may be present in said feedstock in order that the material to be processed will remain in a fluid state during the processing operation and will not cake thereby necessitating treatment in the apparatus which is utilized for the process. Another advantage for the drying operation is that the evolution of water will not occur during the subsequent halgenation step to such an extent which is great enough to form significant quantities of hydrogen halide such as hydrogen chloride, hydrogen bromide, etc., or other detrimental products which could effect either the chemical or physical parameters of the process. The drying of the feedstock may be effected at elevated temperatures ranging from about 100.degree. to about 150.degree. C. for a period of time which is sufficient to reduce the water content of the feed to a value of 2% or less.
Following the aforesaid activation of the concentrate or upon completion of the drying of the feedstock and reduction of the water contained, the lead source is then subjected to a halogenation step in which the concentrate is subjected to the action of a halogenating agent such as chlorine gas, fluorine gas, bromine gas, etc., at a temperature of from about 80.degree. to about 120.degree. C. for a period of time which is sufficient to insure conversion of the lead sulfide to the desired lead halide, said halogenating step preferably being effected in a dry atmosphere; the term "dry atmosphere" being defined as an atmosphere wherein the water content of both the atmosphere and the charge stock combined is not greater than about 0.5%. The treatment of the lead sulfide with the halogenating agent is accomplished in such a manner such as by stirring, mixing, shaking, or by any other means known in the art whereby all of the lead sulfide is contacted with the halogenating agent. The resulting mixture of elemental sulfur which has been formed by the chlorination step along with the lead halide such as lead chloride, lead bromide, or lead fluoride is then subjected to a leaching step. This is effected by treating the resulting mixture with a brine solution at an elevated temperature usually in the range of from about 80.degree. to about 120.degree. C., said brine solution usually comprising an aqueous sodium chloride solution containing from about 20 to about 35% by weight of sodium chloride. The leaching of the mixture usually will take place for a period of time which may range from about 0.5 up to about 2 hours or more in duration, the residence time being that which is sufficient to dissolve the aforesaid lead halide.
Upon completion of the leaching step the solution is filtered while maintaining the temperature of the solution at an elevated range of from about 80.degree. to about 120.degree. C. in order to maintain the lead halide in a soluble form. It is also contemplated within the scope of this invention that the separation of the soluble lead halide and the element sulfur which is in solid form may also be effected by a flotation step or by settling whereby, after allowing the solid residue to settle, the liquid is removed by conventional means such as decantation, etc. The solid sulfur and residue which contains gangue, unreacted sulfides of the impurity metals such as zinc sulfide, copper sulfide, silver sulfide, and iron sulfide, may be subjected to a recovery treatment. For example, the elemental sulfur may be recovered by a froth flotation method in which the sulfur is preferentially floated. Likewise, a scrubbing step to more fully liberate sulfur from the rest of the residue may also be effected in the presence of a flotation promoter such as organic compounds readily available including kerosene, etc. The treated material is then transferred to a flotation cell, a frothing agent is added, aeration is initiated, and the sulfur-laden froth is removed from the cell. As an alternative method for the recovery of sulfur, the residue may also be treated with aqueous ammonium sulfide in which the ammonium polysulfide which is formed permits the recovery of elemental sulfur in a crystalline form or, if so desired, the impurities present in the lead sulfide concentrate may also be recovered by conventional means which will include cyanidation of the residue in a leaching operation to recover silver or other precious metals.
The filtrate which contains the soluble lead halide is then passed to a crystallization zone. Inasmuch as temperature is an important factor in the solubility of lead halide, the soluble lead halide source is maintained at elevated temperatures, preferably in a range of from about 100.degree. to about 150.degree. C., until it is passed into the crystallization zone. The crystallization zone is preferably maintained at a temperature somewhat lower than the leach and separation temperatures and preferably in a range of from about 60.degree. C. to ambient temperature (20.degree.-25.degree. C.) or lower, wherein the soluble lead halide will crystallize out due to the temperature drop. If temperatures lower than ambient are required, the lower temperatures may be obtained by the utilization of external cooling means such as an ice bath, cooling coils, other heat exchangers, etc.
The thus crystallized lead halide is recovered from the crystallization zone and separated from the barren leach solution, the latter, if so desired, then being recycled back to the leaching step for further use therein. The separated crystallized lead halide is then passed to a carbonation zone wherein the lead halide is treated with sodium bicarbonate to form lead carbonate. In the preferred embodiment of the invention, the conversion of the lead halide is effected in a chloride system or a water system. The conversion of the lead halide to lead carbonate may be effected at a temperature in the range of from about 5.degree. to about 95.degree. C. for a period of time ranging from about 0.1 up to about 4 hours or more in duration. At the end of this time the lead carbonate which has been formed is then passed from the carbonation zone to a dissolution zone wherein the product is treated with hydrofluosilicic acid to solubilize the lead as lead fluosilicate. The treatment of the lead halide with the sodium carbonate will, as hereinbefore set forth, result in the formation of lead carbonate. The sodium bicarbonate reacts with the lead halide to form the lead carbonate and also results in the formation of sodium halide, hydrogen halide, along with some carbon dioxide from reaction with a part of the hydrogen halide. The first two in aqueous form are passed to a chloride process brine cell, one example being a Hooker type cell in which the solution is subjected to electrical energy whereby halogen gas is produced and recycled to the halogenation zone, the sodium hydroxide which is formed in the brine cell is passed to a formation zone wherein it is contacted with the carbon dioxide from the dissolution and conversion zones to form sodium bicarbonate which is then recycled back to the conversion zone. The treatment of the lead carbonate with aqueous hydrofluosilicic acid, in which the hydrofluosilicic acid is present in an amount in the range of from about 5 to about 30% by weight is effected at a temperature in the range of from about 10.degree. to about 100.degree. C. After completion of the formation of lead fluosilicate it is then passed into an electrowinning cell wherein the solution is subjected to an electrowinning reaction utilizing standard anodes and cathodes consisting of graphite, titanium, platinum, etc. The electrowinning of the lead fluosilicate is effected under electrolysis conditions in which the parameters which are employed will include voltages within the range of from about 2.5 to about 4.0 volts and current densities in the range of from about 100 to about 400 amps/m.sup.2. The electrowinning of the lead fluosilicate will result in the obtention of metallic lead in the form of sheets on the cathodes, said sheets being dense and coherent in nature. The hydrofluosilicic acid which is formed during the electrolysis reaction may then be recycled back to the dissolution zone for use therein as the dissolution agent.